FIG. 1(a) is a cross sectional view at a key portion of a permanent magnet field small DC motor relating to the present invention; FIG. 1(b) shows an arc-shaped magnet used in the motor. In the drawings, a pair of arc-shaped magnets 1, a soft-magnetic frame 2, an armature 3, which includes a brush-rectifier, a rotating shaft and a bearing, and a U-shaped spring 4 for pressing and securing the pair of arc-shaped magnets 1 in the soft-magnetic frame 2 are shown. The permanent magnet field small DC motor under discussion is requested to be further miniaturized, yet to provide a higher output and an accurate revolving performance, like in other permanent magnet motors.
As a general rule among the permanent magnet field small DC motors, it is difficult to maintain the output with a diameter of the armature 3 reduced. Especially in a motor using a ferrite magnet, whose maximum energy product [BH] max is low irrespective of whether it is fabricated by sintering, or by compression molding, injection molding or extrusion molding of a material mixed with a resin binder, the air-gap between the arc-shaped magnet 1 and an armature 3 is not provided with a sufficiently strong static magnetic field when miniaturized. Hence, the output is significantly reduced. In an attempt to provide the air-gap between the arc-shaped magnet 1 and the armature 3 with strong static magnetic fields among the miniaturized motors, a rare-earth magnet in an arc-shape whose maximum thickness is 1 mm or less, which exhibiting a so-called high [BH] max value, has been required.
Regarding the arc-shaped rare-earth magnet whose maximum thickness is 1 mm or less, Japanese Patent Laid-open Publication No. H6-236807 discloses a method for fabricating an arc-shaped rare-earth magnet. The method comprising the steps of pouring a melted mixture of thermo-plastic resin-binder and various kinds of rare-earth iron based magnetic powders, ranging from anisotropic to isotropic, into a mold, and extrusion molding while cooling it below a melting point of the thermo-plastic resin-binder. According to the disclosure, an arc-shaped rear-earth magnet of 0.9 mm thick can be produced at a thickness variation ±30 μm through extrusion molding from a bonded-magnet compound of, for example, isotropic rare-earth iron based melt-spun flakes 95 weight % and a thermo-plastic resin consisting mainly of 12-nylon. However, it is also mentioned that there is a difficulty in conducting a compression molding of rare-earth-based melt-spun flakes together with a resin-binder. In the extrusion molding, the thermo-plastic resin in the melted state has to perform a role of a carrier for the rare-earth iron based melt-spun flakes. So, as compared to a compression molded rare earth magnet, which is prepared from rare-earth iron based melt-spun flakes mixed with a thermosetting resin of normally 3 weight % or less, the content of rare-earth iron based melt-spun flakes needs to be lowered in the magnet of the above disclosure. Accordingly, the [BH] max value of the magnet becomes lower, and the static magnetic fields between the arc-shaped magnet 1 and the armature 3 become weaker.
One of the problems with the permanent magnet field small DC motor comprising the arc-shaped rare earth magnet fabricated through the above extrusion molding, which provides a stronger static magnet field in an air-gap to the armature 3 as compared to a ferrite magnet motor, is an increased cogging torque. This is the torque pulsation due to permeance changes as a result of revolution of the armature 3 caused by iron core teeth 31 and a slot 32 existing on the outer circumferential surface of the armature 3 disposed opposing to the arc-shaped magnet 1. The cogging torque is a substantial problem among the permanent magnet field small DC motors which are expected to be compact in size, yet required a high mechanical output with an accurate revolving performance. The motors are the object of the present invention.
Among the motors having an arc-shaped magnet, regardless whether it is a rare earth magnet or not, known technologies for reducing the cogging torque through a shape of the arc-shaped magnet include making a radius of the outer surface of the arc-shaped magnet to be different from that of the inner surface, or cutting edges at both ends in the circumferential direction of an arc-shaped magnet, thereby making a distribution of flux density in the air-gap closer to a sine curve (an example of a publication: Shogo Tanaka, “Application of Permanent Magnets for Small Motors”, page 7 in the proceedings of the Symposium of Small Motor Technology, 1983). Japanese Utility Model Publication No. S49-4651 discloses that, in a permanent magnet field small DC motor, a cut provided in an arc-shaped magnet in the outer surface at both sides off of the center of the magnetic pole suppresses reduction of effective flux at the center of magnetic pole, despite the reduction at the cut portion. Although there is no mention about the cogging torque in the Utility Model, there is an indication about a possibility that the cut provided in an arc-shaped magnet in the outer surface at both sides off of the center of the magnetic pole would reduce the cogging torque in a permanent magnet field small DC motor, while controlling deterioration of the rotating torque.
A practical means for reducing the cogging torque in a permanent magnet field small DC motor, including the use of a thin arc-shaped rare earth magnet, is disclosed in Japanese Patent Laid-open Publication No. H11-18390. The disclosed means has a relevance with a method shown in FIGS. 2(a), (b). In the drawings, a pair of arcuate permanent magnets 1, a soft-magnetic frame 2, an armature 3 including a brush-rectifier, a rotating shaft and a bearing, and hooking protrusions 21 for fitting and securing the pair of arc-shaped permanent magnets 1 to the soft-magnetic frame 2 are shown.
According to the disclosure, (1) the radius curvature of an arc-shaped magnet 1 at the outer surface is deviated from that of the inner surface so that the air-gap between the magnet and an armature core increases along with the increasing distance along the direction of circumference from the center of a magnetic pole towards both ends, namely, a structure of so-called uneven air-gap, (2) the arc-shaped magnet having deviated radius curvature in the outer surface and inner surface is provided with a cut portion 11 so that there is an air-gap between the arc-shaped magnet and the soft-magnetic frame 2, and (3) the arc-shaped magnet 1 is slightly bent and fitted to be secured in the soft-magnetic frame 2 between the hooking protrusions 21. (An arc-shaped rare earth magnet with 12-nylon fabricated by extrusion molding, reference the Japanese Patent Laid-open Publication No. H6-236807, can be slightly bent.)
The direction of thrust in the arc-shaped rare earth magnet 1 coincides with that of extrusion in the extrusion molding, so the cross sectional shapes in a direction remain identical as shown in FIG. 2(b). Therefore, when an arc-shaped rare-earth magnet 1 is secured in a soft-magnetic frame 2 with both ends thinned in the circumferential direction of the magnet 1, at which a vacant space is formed to the soft-magnetic frame 2, engaged by the hooking protrusions 21, the bent quantity at magnet 1 might increase depending on the variation in thickness value ±30 m in the magnet 1. This can lead to a crack, tip-cut, etc. of the magnet 1 at the engagement portion, or the magnet 1 even might drop off the frame. Thus, there is a possibility of serious hazards in the reliability of the permanent magnet field small DC motors in the above disclosure.
Next, a practical means for fixing a thin arc-shaped rare earth magnet to a soft-magnetic frame of permanent magnet field small DC motors is disclosed in Japanese Patent Laid-open Publication No. H11-18390, which is shown in FIGS. 3(a), (b).
In the drawings, a pair of arc-shaped permanent magnets 1, a portion 11 of magnet for reducing cogging torque, engagement portions 12 of the magnet, a soft-magnetic frame 2, an armature 3 including the brush-rectifier, shaft and bearing, and hooking protrusions 21 for fixing and securing the pair of arcuate permanent magnets 1 in the soft-magnetic frame 2 are shown.
According to the disclosure, an arc-shaped rare earth magnet 1 has two or more different shapes in the cross section along the thrust direction; namely, a shape of the engagement portion 12 which is to be fitted in between the hooking protrusions 21 of the soft-magnetic frame 2 and a shape of the portion 11 for reducing cogging torque. The means for reducing cogging torque disclosed in the above Laid-open patent remains identical to that taught in the Japanese Patent Laid-open Publication No. H10-201206, which is generally known means like deviating curvatures or cutting corners of a magnet. The problems of a crack, tip-cut, etc. of an arc-shaped rare earth magnet 1 at the engagement portion that might arise when it is fixed and secured in a soft-magnetic frame 2, or a drop-off of a magnet 1, which are relevant to the Japanese Patent Laid-open Publication No. H10-201206, can be significantly improved.
However, there is a difficulty in fabricating by, for example, extrusion molding an arc-shaped rare earth magnet of maximum thickness 1 mm or less that has two or more shapes in the cross section along the thrust direction. Providing the portion 11 for reducing the cogging torque will need a post machining process of cutting off both ends in the circumferential direction. Cutting a thin arc-shaped rare earth magnet 1 at a high precision level is quite difficult work, and cracks and drop-offs readily arise to invite a poor manufacturing yield rate. Furthermore, particles of the raw material of rare earth magnet may increase the difficulty in finishing an arc-shaped the rare earth magnet 1 to be ready for mounting in a soft-magnetic frame 2.
A rare earth magnet fabricated by compression molding from isotropic rare earth iron based melt-spun flakes mixed with a binder (e.g., an epoxy resin) for 1.5–3.0 weight %, in general, and heated for curing the binder, exhibits a density of 5.8–6.1 g/cm3. Whereas, the magnet fabricated by extrusion molding from the same rare earth iron based melt-spun flakes, which needs a binder (e.g., 12-nylon) for more than 5 weight %, exhibits a density of 5.7 g/cm3 or less. Since the maximum energy product [BH] max of the magnet is dependent on the quantity of the filled rare earth iron based melt-spun flakes, or the magnet density, a compression molded rare earth magnet that can offer a higher [BH] max is more advantageous for providing a strong static magnet field in the air-gap between an armature and a magnet in the permanent magnet field small DC motors, as compared with the one fabricated by extrusion molding.
A first problem to be solved by the present invention is the problem described in Japanese Patent Laid-open Publication No. H6-236807, meaning that “since there is a substantially wide variation in the weighing at molding, it has been considered to be difficult to provide by compression molding thin arc-shaped magnets having a maximum thickness of 1 mm or less at a dimensional accuracy ±30 μm”. Even if the problem is solved, however, the mechanical strength of the compression molded rare earth magnet is low at the room temperature area, and brittle, because of a lower amount of resin contained therein. So, the arc-shaped rare earth magnets fabricated by compression molding are not suitable for fixing and securing in a soft-magnetic frame in accordance with the method taught in the Japanese Patent Laid-open Publication No. H10-201206 and the Japanese Patent Laid-open Publication No. H11-18390, meaning that attaching a magnet “at the engagement portion between the hooking protrusions of soft-magnetic frame with the magnet slightly bent”. Namely, a second problem to be solved by the present invention is to contrive new means for securing an arc-shaped rare earth magnet in a soft-magnetic frame by appropriately taking the physical properties of the magnet into consideration. In addition, since “the [BH] max is higher than that of an arc-shaped rare earth magnet fabricated by extrusion molding”, a third task to be attained in the present invention is to offer new additional means for controlling the distribution of flux density in the air-gap by a known means of providing the magnet with an appropriate shape for reducing the cogging torque.